1. Field of the Invention
The present invention relates to real-time phase measurement of an electrical signal. The invention has particular application to phase measurements when measuring acoustic pressure field variations with a sonar transducer.
2. Description of the Related Art
Two methods are commonly used to measure an acoustic pressure with a sonar transducer. The first is to record in an analog fashion the output of the transducer for subsequent digitization and processing on a digital computer. The other method is to use real-time instrumentation for retrieval of the amplitude and phase. The latter method is preferred because errors while conducting an experiment can more easily be detected and corrected. Continuous wave phase meters, such as the Hewlett-Packard Model HP-3575A, have been used to measure the transducer signal phase relative to a reference signal. These devices are designed for electronic bench testing of analog circuits where noisy signals are of no concern. Also, these devices have response times which prevent them from following rapidly varying phases. When short pulses of continuous-wave signals (SONAR) are used, the measurement of phase is complicated in that the pertinent phase information is contained in a shorter period of time. Sampling continuous-wave phase meters are available, such as Model No. VIP-100M, manufactured by Ocean Data Equipment Corporation, which can be used on signals for which the pertinent phase information is contained in a short period of time. However, such phase meters do not provide accurate measurements in the presence of noise. U.S. Pat. No. 3,878,526 to Pedersen discloses phase measurement in a signal processor employing quadrature reference signals to detect target movement. U.S. Pat. No. 3,646,34 to Wold discloses a hybrid analog/digital system which correlates two input signals. U.S. Pat. No. 4,071,821 to Harthill et al. discloses a digital apparatus for determining the phase of a signal by quadrature correlation. The Harthill patent does not appear to disclose any noise compensation such as pulse-to-pulse averaging of the phase count.
A digital FFT analyzer, such as Model No. 556, manufactured by NICOLET, has the capability of obtaining the phase of short-duration signals against a noisy background. However, the FFT analyzer, since it also performs more complex operations on a signal, is a large and complex piece of equipment.
Thus, there is a need in the art for a simple device which can be used when phase measurements alone are required of a pulsed single-frequency signal in a background of noise.